Russia has been moving artillery units to areas of northern Syria where government forces have massed, raising U.S. concern the two allies may be preparing for a return to full-scale fighting as the current cease-fire falters, U.S. officials said.

The recent Russian redeployments of the units and the forces that operate them have been accompanied by the return of some Iranian army forces to government-controlled areas close to the front lines, according to officials briefed on the intelligence. Russia, Iran and the...